Air/fuel control method with adaptive feedback actuation

ABSTRACT

An engine air/fuel control system includes an apparatus and method for adaptively setting an initialization period preceding closed loop fuel control. During the initialization period, fuel delivered to the engine is modulated by a periodic waveform. A high voltage signal associated with the high output state of an exhaust gas oxygen sensor and a low voltage signal associated with the low voltage state of the exhaust gas oxygen sensor are sampled. When the difference between these signals exceeds a preselected value, the initialization period is terminated and closed loop fuel control is actuated.

BACKGROUND OF THE INVENTION

The field of the invention relates to control systems for controllingengine air/fuel operation in response to exhaust gas sensors.

U.S. Pat. No. 4,132,200 discloses a feedback control system in which afeedback signal is generated by comparing an exhaust gas oxygen sensoroutput to a reference signal. The reference signal is generated by timeaveraging the sensor output. During open loop control, fuel is deliveredin relation to a fuel signal which is biased rich and dithered. When anaverage in the number of sensor output transitions beyond the referencevalue exceeds a threshold, feedback control is initiated.

The inventors herein have recognized numerous problems with the aboveapproaches. For example, the switch from open loop to feedback controlmay be delayed beyond the time at which the exhaust gas oxygen sensor issufficiently heated to fully operable. This delay occurs because twoaverages are needed requiring numerous cycles of the sensor output. Oneaverage is required to generate the reference signal, and anotheraverage of the comparison between sensor output and the reference isalso required. The need remains to more quickly and accurately determinewhen the sensor becomes fully operable and feedback control iscommenced.

SUMMARY OF THE INVENTION

An object of the invention herein is to learn when the exhaust gasoxygen sensor becomes operable and initiate feedback control at thattime.

The above object is achieved, and problems of prior approaches overcome,by providing an engine air/fuel control method and control systemresponsive to an output from an exhaust gas sensor. In one particularaspect of the invention, the method comprises the steps of: modulatingfuel delivered to the engine during an initialization period;terminating the initialization period when a difference between high andlow excursion in the sensor output exceeds a preselected value; andadjusting fuel delivered to the engine in response to a feedbackvariable derived from the sensor output, the adjusting step beinginitiated in response to the termination of the initialization period.

An advantage of the above aspect of the invention is that feedbackair/fuel control is actuated at the time the exhaust gas sensorcommences desired operation. Another advantage is that theinitialization period, typically occurring after engine start or atransient engine operating condition, is minimized thereby minimizingengine emissions.

In another aspect of the invention, the control method comprises:modulating fuel delivered to the engine during an initialization period;generating a first signal by storing the sensor output as the firstsignal while the sensor output is greater than a previously stored firstsignal and decreasing the previously stored first signal at apredetermined rate while the sensor output is less than the previouslystored first signal; generating a second signal by storing the sensoroutput as the second signal while the sensor output is less than apreviously stored second signal and increasing the previously storedsecond signal at a predetermined rate while the sensor output is greaterthan the previously stored second signal; terminating the initializationperiod when a difference between the first signal and the second signalexceeds a preselected value; and adjusting fuel delivered to the enginein response to a feedback variable derived from the sensor output, theadjusting step being initiated in response to the termination of theinitialization period.

An advantage of the above aspect of the invention is that the durationof the initialization period is adaptively learned so that feedbackcontrol commences at the approximate time the exhaust gas sensor issufficiently warmed to commence feedback control.

BRIEF DESCRIPTION OF THE DRAWINGS

The above object and advantages of the invention claimed herein andothers will be more clearly understood by reading an example of anembodiment in which the invention is used to advantage with reference tothe attached drawings wherein:

FIG. 1 is a block diagram of an embodiment in which the invention isused to advantage;

FIGS. 2-5 are high level flowcharts illustrating various steps performedby a portion of the embodiment illustrated in FIG. 1;

FIGS. 6A, 6B, 7, and 8 illustrate various outputs associated with aportion of the embodiment illustrated in FIG. 1 and explained withreference to the flowcharts shown in FIGS. 2-5;

FIG. 9 is a high level flowchart illustrating various steps performed bya portion of the embodiment illustrated in FIG. 1; and

FIGS. 10-11 illustrate various outputs associated with a portion of theembodiment illustrated in FIG. 1 and explained herein with particularreference to FIG. 9.

DESCRIPTION OF AN EMBODIMENT

Controller 10 is shown in the block diagram of FIG. 1 as a conventionalmicrocomputer including: microprocessor unit 12; input ports 14including both digital and analog inputs; output ports 16 including bothdigital and analog outputs; read only memory (ROM) 18 for storingcontrol programs; random access memory (RAM) 20 for temporary datastorage which may also be used for counters or timers; keep-alive memory(KAM) 22 for storing earned values; and a conventional data bus.

In this particular example, exhaust gas oxygen (EGO) sensor 34 is showncoupled to exhaust manifold 36 of engine 34 upstream of conventionalcatalytic converter 38. Tachometer 42 and temperature sensor 40 are eachshown coupled to engine 24 for providing, respectively, signal rpmrelated to engine speed and signal T related to engine coolanttemperature to controller 10.

Intake manifold 44 of engine 24 is shown coupled to throttle body 46having primary throttle plate 48 positioned therein. Throttle body 46 isalso shown having fuel injector 50 coupled thereto for delivering liquidfuel in proportion to pulse width signal fpw from controller 10. Fuel isdelivered to fuel injector 50 by a conventional fuel system includingfuel tank 52, fuel pump 54, and fuel rail 56.

Referring now to FIG. 2, two-state signal EGOS is generated by comparingsignal EGO from sensor 34 to adaptively learned reference value Vs. Morespecifically, when various operating conditions of engine 24, such astemperature (T), exceed preselected values, closed-loop air/fuelfeedback control is commenced (step 102). Each sample period ofcontroller 10, the output of sensor 34 is sampled to generate signalEGO_(i). Each sample period (i) when signal EGO_(i) is greater thanadaptively learned reference or set voltage Vs_(i) (step 104), signalEGOS_(i) is set equal to a positive value such as unity (step 108). Onthe other hand, when signal EGO_(i) is less than reference value Vs_(i)(step 104) during sample time (i), signal EGOS_(i) is set equal to anegative value such as minus one (step 110). Accordingly, two-statesignal EGOS is generated with a positive value indicating exhaust gasesare rich of a desired air/fuel ratio such as stoichiometry, and anegative value when exhaust gases are lean of the desired air/fuelratio. In response to signal EGOS, feedback variable FFV is generated asdescribed later herein with particular reference to FIG. 4 for adjustingthe engine's air/fuel ratio.

A flowchart of the liquid fuel delivery routine executed by controller10 for controlling engine 24 is now described beginning with referenceto the flowchart shown in FIG. 3. An open loop calculation of desiredliquid fuel is first calculated in step 300. More specifically, themeasurement of inducted mass airflow (MAF) from sensor 26 is divided bya desired air/fuel ratio (AFd). After a determination is made thatclosed loop or feedback control is desired (step 302), the open loopfuel calculation is trimmed by fuel feedback variable FFV to generatedesired fuel signal fd during step 304. This desired fuel signal isconverted into fuel pulse width signal fpw for actuating fuel injector50 (step 306) via injector driver 60 (FIG. 1).

As described in greater detail later herein with particular reference toFIG. 9, desired fuel signal fd is modulated (step 308) by a periodicsignal during an initialization period. Any periodic signal may be usedsuch as a triangular wave, sine wave, or square wave. Thisinitialization period precedes and is preparatory to closed loopfeedback control.

The air/fuel feedback routine executed by controller 10 to generate fuelfeedback variable FFV is now described with reference to the flowchartshown in FIG. 4. After closed control is commenced (step 410), signalEGOS_(i) is read during sample time (i) from the routine previouslydescribed with respect to steps 108-110. When signal EGOS_(i) is low(step 416), but was high during the previous sample time or backgroundloop (i-1) of controller 10 (step 418), preselected proportional term Pjis subtracted from feedback variable FFV (step 420). When signalEGOS_(i) is low (step 416), and was also low during the previous sampletime (step 418), preselected integral term Δj is subtracted fromfeedback variable FFV (step 422).

Similarly, when signal EGOS is high (step 416), and was also high duringthe previous sample time (step 424), integral term Ai is added tofeedback variable FFV (step 426). When signal EGOS is high (step 416),but was low during the previous sample time (step 424), proportionalterm Pi is added to feedback variable FFV (step 428).

Adaptively learning set or reference Vs is now described with referenceto the subroutine shown in FIG. 5. For illustrative purposes, referenceis also made to the hypothetical operation shown by the waveformspresented in FIGS. 6A and 6B. In general, adaptively learned referenceVs is determined from the midpoint between high voltage signal Vh andlow voltage signal V1. Signals Vh and V1 are related to the high and lowvalues of signal EGO during each of its cycles with the addition ofseveral features which enables accurate adaptive learning underconditions when signal EGO may become temporarily pegged at a richvalue, or a lean value, or shifted from its previous value.

Referring first to FIG. 5, after closed loop air/fuel control iscommenced (step 502), signal EGO_(i) for this sample period (i) iscompared to reference Vs_(i-1) which was stored from the previous sampleperiod (i-1) in step 504. When signal EGO_(i) is greater than previouslysampled signal Vs_(i) -1, the previously sampled low voltage signal.Vl_(i) -1 is stored as low voltage signal Vl_(i) for this sample period(i) in step 510. This operation is shown by the graphical representationof signal V1 before time t2 shown in FIG. 6A. Returning to FIG. 5, whensignal EGO_(i) is greater than previously sampled high voltage signalVh_(i-1) (step 514), signal EGO_(i) is stored as high voltage signalVh_(i) for this sample period (i) in step 516. This operation is shownin the hypothetical example of FIG. 6A between times t1 and t2.

When signal EGO_(i) is less than previously stored high voltage signalVh_(i-1) (step 514), but greater than signal VS₁₋₁, high voltage signalVh_(i) is set equal to previously sampled high voltage Vh_(i-1) lesspredetermined amount D_(i) which is a value corresponding to desiredsignal decay (step 518). This operation is shown in the hypotheticalexample presented in FIG. 6A between times t2 and t3. As shown in FIG.6A, high voltage signal Vh decays until signal EGO_(i) falls to a valueless than reference Vs at which time high voltage signal Vh is heldconstant. Although linear decay is shown in this example, nonlineardecay and experiential decay may be used to advantage. Referring to thecorresponding operation shown in FIG. 5, high voltage signal Vh_(i) isstored as previously sampled high voltage signal Vh_(i-1) (step 520)when signal EGO_(i) is less than previously sampled reference Vs_(i-1)(step 504).

Continuing with FIG. 5, when signal EGO_(i) is less than both previouslysampled reference Vs_(i-1) and previously sampled low voltage signalV1_(i-1) (step 524) signal EGO_(i) is stored as low voltage signalV1_(i) (step 526). An example of this operation is presented in FIG. 6Abetween times t4 and t5.

When signal EGO_(i) is less than previously sampled reference Vs_(i-1)(step 504), but greater than previously sampled high voltage signalVl_(i-1) (step 524), high voltage signal Vl_(i) is set equal topreviously sampled high voltage signal Vl_(i) -1 plus predetermineddecay value D_(i) (step 530). The decay applied in step 530 may bedifferent from that applied in step 518. An example of this operation isshown graphically in FIG. 6A between times t5 and t6.

As shown in step 532 of FIG. 5, reference Vs_(i) is calculated eachsample period (i) by interpolating between high voltage signal Vh_(i)and low voltage signal V1_(i) each sample time (i) represented by Vs=(δVh1+(1-d) Vli)/2. In this particular example, a midpoint calculation isused to advantage.

Referring to the hypothetical example presented in FIGS. 6A and 6B,signal EGOS is set at a high output amplitude (+A) when signal EGO isgreater than reference Vs and set at a low value (-A) when signal EGO isless than reference Vs.

In accordance with the above described operation, reference Vs isadaptively learned each sample period so that signal EGOS is accuratelydetermined regardless of any shifts in the output of signal EGO. Inaddition, advantageous features such as allowing high voltage signal Vhand low voltage signal V1 to decay only to values determined by the zerocrossing point of signal EGO, prevent the reference from becomingtemporarily pegged when air/fuel operation runs rich or lean forprolonged periods of time. Such operation may occur during eitherwide-open throttle conditions or deceleration conditions.

Advantages of the above described method for adaptively learningreference Vs are shown in FIGS. 7 and 8 during conditions where signalEGO incurs a sudden shift. More specifically, FIG. 7 shows ahypothetical operation wherein high voltage signal Vh and low voltagesignal V1 accurately track the outer envelope of signal EGO and theresulting reference is shown accurately and continuously tracking themidpoint in peak-to-peak excursions of signal EGO in FIG. 8.

An initialization period having an adaptively learned period or timeduration which precedes closed loop fuel control is now described withreference to the flowchart shown in FIG. 9 and related waveforms shownin FIGS. 10 and 11. In general, during the initialization period, openloop fuel control is modulated by superimposing a periodic signal on thedesired fuel charge signal. When a form of the modulation is detected inthe output of EGO sensor 34, an indication is provided that EGO sensor34 has achieved proper operation and, accordingly, closed loop fuelcontrol commences. Those skilled in the art will recognize that althoughsensor 34 is shown in this example as a conventional two-state exhaustgas oxygen sensor, the invention described herein is applicable to othertypes of exhaust gas oxygen sensors such as proportional sensors and isalso applicable to other types of exhaust sensors such as HC and NO_(x)sensors.

First referring to FIG. 9, engine operating parameters associated withclosed loop fuel control are first sampled during step 550. In thisexample, these parameters include engine temperature T being beyond apreselected temperature. When the closed loop parameters are absent, theclosed loop flag is reset in step 552 thereby disabling closed loop fuelcontrol. On the other hand, when the closed loop parameters are present,the initializing subroutine is entered provided that engine 24 is notpresently operating in closed loop fuel control (step 556).

Upon entering the initialization period, a modulation signal having aperiodic cycle such as a triangular or sinusoidal wave is firstgenerated during step 558. As previously described herein withparticular reference to FIG. 3, the modulating signal modulates thedesired fuel quantity delivered to engine 24.

Continuing with FIG. 9, when signal EGO_(i) for this sample period (i)is less than low voltage signal Vl_(i-1) stored from the previous sampleperiod (i -1), low voltage signal V1_(i) is set equal to signal EGO_(i)(step 564). On the other hand, when signal EGO_(i) is greater thanpreviously stored signal V1_(i-1) (step 562), signal V1_(i) for thissample period is set equal to previously stored signal Vl_(i-1) pluspredetermined value D_(i) (step 568). In this particular example,predetermined value D_(i) is added when required each sample time togenerate a predetermined rate which is applied to increase or decreasethe signals described herein.

When signal EGO_(i) is less than previously stored high voltage signalVh_(i-1) as shown in step 572, then signal Vh_(i) decays at apredetermined rate as provided by predetermined value D_(i). Morespecifically, as shown in step 576, signal Vh_(i) is set equal topreviously stored signal Vh_(i-1) less predetermined value D_(i).However, when signal EGO_(i) is greater than signal Vh_(i-1). (step572), signal Vh_(i) is set equal to signal EGO_(i) for this sampleperiod (i) as shown in step 578.

The difference between signal Vh_(i) and signal V1_(i) is then comparedto preselected value x during step 582. When this difference exceedspreselected value x, it is apparent that a sufficient portion of theinput modulation is observed at the output of EGO sensor 34 such thatclosed loop fuel control should commence. Accordingly, the closed loopfuel flag is set in step 584.

For illustrative purposes, a hypothetical example is illustrated by thewaveforms in FIG. 10. More specifically, a hypothetical signal EGO isshown and the associated high voltage signal Vh and low voltage signalV1 are illustrated by the waveforms shown in FIG. 10. For the particularexample, there is a sufficient difference between signal Vh and signalVl to terminate the initialization period and actuate closed loopfeedback control.

Another hypothetical operation is illustrated in FIG. 11. In thisparticular example, the initialization period occurs between times t_(O)and t₁. At time t1, the above described input modulation is detected insignal EGO, the initialization period then terminated, and feedbackcontrol commenced.

Although one example of an embodiment which practices the invention hasbeen described herein, there are numerous other examples which couldalso be described. For example, the invention may be used to advantagewith proportional exhaust gas oxygen sensors. Further, othercombinations of analog devices and discrete ICs may be used to advantageto generate the current flow in the sensor electrode. The invention istherefore to be defined only in accordance with the following claims.

What is claimed:
 1. An engine air/fuel control method responsive to anoutput from an exhaust gas sensor, comprising the steps of:modulatingfuel delivered to the engine during an initialization period;terminating said initialization period when a difference between highand low excursion in the sensor output exceeds a preselected value; andadjusting fuel delivered to the engine in response to a feedbackvariable derived from the sensor output, said adjusting step beinginitiated in response to said termination of said initialization period.2. The control method recited in claim 1 wherein said modulating stepincludes the step of modulating a fuel delivery signal.
 3. The controlmethod recited in claim 2 further comprising the step of generating saidfuel delivery signal by dividing a measurement of airflow inducted intothe engine by a desired air/fuel ratio.
 4. The method recited in claim 3wherein said modulating step includes the step of superimposing aperiodic signal on said fuel delivery signal.
 5. The method recited inclaim 4 wherein said periodic signal comprises a triangular wave.
 6. Themethod recited in claim 4 wherein said periodic signal comprises a sinewave.
 7. The control method recited in claim 3 wherein said step ofadjusting fuel delivered to the engine further comprises the steps ofintegrating the sensor output to generate said feedback variable andfurther dividing said inducted airflow measurement by said feedbackvariable.
 8. The method recited in claim 1 further comprising the stepof generating said initialization period in response to an indication ofat least one engine operating parameter exceeding a preselected value.9. The method recited in claim 1 wherein said modulating step and saidfuel adjusting step are terminated in response to an indication of atleast one engine operating parameter falling below a preselected value.10. The method recited in claim 1 wherein said sensor comprises anexhaust gas oxygen sensor.
 11. An engine air/fuel control methodresponsive to an output from an exhaust gas sensor, comprising the stepsof:modulating fuel delivered to the engine during an initializationperiod; generating a first signal by storing the sensor output as saidfirst signal while the sensor output is greater than a previously storedfirst signal and decreasing said previously stored first signal at apredetermined rate while the sensor output is less than said previouslystored first signal; generating a second signal by storing the sensoroutput as said second signal while the sensor output is less than apreviously stored second signal and increasing said previously storedsecond signal at a predetermined rate while the sensor output is greaterthan said previously stored second signal; terminating saidinitialization period when a difference between said first signal andsaid second signal exceeds a preselected value; and adjusting fueldelivered to the engine in response to a feedback variable derived fromthe sensor output, said adjusting step being initiated in response tosaid termination of said initialization period.
 12. The method recitedin claim 11 wherein said fuel adjusting step generates said feedbackvariable by integrating a two-state signal derived from the sensoroutput.
 13. The method recited in claim 12 further comprising the stepof generating said two-state signal by comparing the sensor output to areference value.
 14. The method recited in claim 13 further comprisingthe step of generating said reference value by generating a midpointbetween said first signal and said second signal.
 15. An air/fuelcontrol system for an internal combustion engine, comprising:acontroller maintaining an air/fuel mixture inducted into the engine neara desired air/fuel ratio in response to a feedback variable after aninitialization period; feedback means for generating said feedbackvariable by integrating a two-state signal generated by comparing anoutput from an exhaust gas oxygen sensor to an adaptively learnedreference signal; adaptive learning means for providing said referencesignal by determining a midpoint between a first signal and a secondsignal during each of a repetitively occurring number of sample times;first signal generating means for generating said first signal each ofsaid sample times by storing said sensor signal as said first signalwhen said sensor signal is greater than said first signal from theprevious sample time and decreasing said first signal by a predeterminedamount when said sensor signal is greater than said previously sampledreference signal but less than said previously sampled first signal;second signal generating means for generating said second signal each ofsaid sample times by storing said sensor signal as said second signalwhen said sensor signal is less than said second signal from theprevious sample time and increasing said second signal by apredetermined amount when said sensor signal is less than saidpreviously sampled reference signal but greater than said previouslysampled second signal; modulation means for modulating fuel delivered tothe engine during said initialization period; and initialization meansfor generating said initialization period in response to an indicationof at least one engine operating parameter exceeding a preselected valueand terminating said initialization period when a difference betweensaid first signal and said second signal exceeds a preselected value.16. The control system recited in claim 15 wherein said first signalgenerating means further comprises means for holding said first signalwhen said sensor signal is less than said reference signal from theprevious sample time.
 17. The control system recited in claim 16 furthercomprising means for holding said second signal when said sensor signalis greater than said reference signal from the previous sample time.